.photo {
    width: 500px;
    height: 300px;
    border: 1px solid black;
/*    设置固定定位*/
    position: fixed;
    left: 50%;
    top: 50%;
    margin-left: -251px;
    margin-top: -151px;


    background: white;

/*    居中*/

    z-index: 999;

}


.mask {
    position: fixed;
    width: 100%;
    height: 100%;
    left: 0px;
    top: 0px;
    background: #172c45;

    z-index: 998;
    opacity: 0.8;


}


